A simple calculation taking the specifications (29"L x 14"W x 11"H) from Earthbox site gives more than 16 dry gallons:

29x14x11=4466 cubic inches

Using cubic inches to dry gallon converter gives:

4466 inÃ¯Â¿Â½ = 16.6144 gal(US Dry)

Some volume should be subtracted because of the water reservoir at the bottom of the container. On the other hand, Earthbox instruction recommens making a 2" mound of the potting medium, so 16 gallon should be very close to the actual volume.

I had a similar concern, so I am trying four pepper plants in one and two determinate tomato plants in another. I have used the original self watering containers from gardener's supply for years, and they work great with one pepper plant in each.